kbuild: rust_is_available: check that environment variables are set

Sometimes [1] users may attempt to setup the Rust support by
checking what Kbuild does and they end up finding out about
`scripts/rust_is_available.sh`. Inevitably, they run the script
directly, but unless they setup the required variables,
the result of the script is not meaningful.

We could add some defaults to the variables, but that could be
confusing for those that may override the defaults (compared
to their kernel builds), and `$CC` would not be a simple default
in any case.

Therefore, instead, explicitly check whether the expected variables
are set (`$RUSTC`, `$BINDGEN` and `$CC`). If not, print an explanation
about the fact that the script is meant to be called from Kbuild,
since that is the most likely cause for the variables not being set.

Link: https://lore.kernel.org/oe-kbuild-all/[email protected]/ [1]
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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